1. Field of the Invention
The present invention relates to a multi-color offset printing method and a printing press for printing a multi-colored image pattern by using a plurality of color inks, and particularly to such a method and apparatus which can shorten the rising time necessary for stabilizing printing conditions from the beginning of printing.
2. Prior Art
Known offset printing presses used for multi-color offset printing are operated with the following fundamental processes.
(1) Each printing plate is damped. PA1 (2) A plurality of color printing inks are supplied to an image area of each of the corresponding printing plates by inking arrangements. PA1 (3) An ink (hereinafter referred to "a color pattern ink") supplied to an image area of each plate is transferred to respective corresponding blankets in a predetermined order. PA1 (4) The color pattern ink transferred to the blankets is overlapped on a sheet of paper so that each of the color images may be properly aligned to obtain a desired color print.
As described above, conventional offset printing utilizes a printing method in which inks are not transferred directly to paper from the plates, but transferred to paper through a blanket. Hereinafter, outlines of the mechanism and operation of several kinds of multi-color offset presses which carry out offset printing are described.
FIG. 10 is a sectional view of a four color offset proofing press for carrying out a continuous printing operation on sheet paper (hereinafter a press of this type is referred to as "a rotary press"), which is described in the specification of U.S. Pat. Nos. 3,536,006 and 3,347,160. The apparatus comprises a plate cylinder (103) on the outer circumference thereof; printing plates (101) (101b) (101c) and (101d) respectively for four colors being provided in a required order; a blanket cylinder (107) on the outer circumference of which four blankets (105a) (105b) (105c) and (105d) having the same diameter as that of the plate cylinder (103) and corresponding to each of the colors being provided in a required order; and a printing cylinder (109) having a diameter of one fourth of that of the plate cylinder (103) and the blanket cylinder (107), and on the outer circumference of which papers are to be set. Here, each of the alpha suffixes attached to the reference number (i.e., a, b, c and d) represents a respective color of printing ink applied to the corresponding parts and also the printing order; and a reference number to which no suffix is attached (e.g. 103) indicates the part in generic.
The apparatus shown in FIG. 10 is arranged such that, during one revolution of the plate cylinder (103) in the direction shown by an arrow mark, water is supplied to each of plates (101a) (101b) (101c) and (101d) from respective damping devices (113a) (113b) (113c) and (113d) to damp each of the plates. Then, from each of inking arrangements (111a) (111b) (111c) and (111d) corresponding to respective colors a color ink is supplied to each of the plates to form a color image on the respective plates. During one revolution of the blanket cylinder (107) which rotates in contact with the plate cylinder (103), the color images on the plates are transferred to the corresponding blankets (105a) (105b) (105c) and (105d), respectively.
On the other hand papers are fed from a paper feeder (115) to the outer circumference of the printing cylinder (109), and, during one revolution of the blanket cylinder (107), the printing cylinder (109) in contact with the blanket cylinder (107) rotates four turns, and inks of the color images on the blanket are overlapped on the paper and transferred thereto. The paper on which four color inks are printed is fed to a paper receiving tray (117).
An apparatus shown in FIG. 11 is another rotary type four color offset proofing press, which is described on pp. 47-50 in a publication entitled "Duetcher Drucker Nr. 33/18-10-1984". This apparatus comprises four plate cylinders (121a) (121b) (121c) and (121d), each of which has one of four printing plates on its outer circumference; four blanket cylinders (123a) (123b) (123c) and (123d) having the same diameter as that of the plate cylinders, each of the outer circumference of them being provided with a blanket and each of them being rotated in contact with the corresponding plate cylinders, respectively; and a printing cylinder (125) having a diameter of about three times that of the plate cylinders and the blanket cylinders. To each of the plate cylinders (121a) (121b) (121c) and (121d) there are attached inking arrangements and a damping device, respectively.
The apparatus of FIG. 11 is of the same type as that shown in FIG. 10, and forms color images on the printing plates by feeding an ink from each inking arrangement to a respective corresponding plate; and the apparatus then prints each of the color pattern inks on the same area of a paper through each of the blankets. According to the rotations of the printing cylinder (125), a sheet of paper fed from a paper feeder (127) passes through points at which each of the blanket cylinders (123a) (123b) (123c) and (123d) contacts the printing cylinder (125) in order; and the paper color images of the respective color inks are overlapped and thus printed. The paper on which a printing of four colors is made is fed to a receiving device (129).
FIG. 12 shows a four color offset proofing press disclosed in British Patent Laid-Open Publication No. 2164295A. The apparatus disclosed in this publication is of a different type from the above-described two rotary type apparatus, and includes a flat table type on which printing plates are loaded planarly. The proofing press of FIG. 12 is provided with a frame (146) having tables or beds (141) (142) (143) and (144) on which printing plates of each color are to be loaded; a set of damping devices (166) and inking arrangements (181) (182) (183) and (184) for each color. In addition, in a carriage (148) which travels on the frame (146) there are provided a blanket cylinder (150) on the outer circumference of which four blankets (151) (152) (153) and (154) are loaded; a printing cylinder (158) having a diameter of one fourth of that of the blanket cylinder (150); water supplying rollers (160) for supplying damping water to a water distributing plate (168); damping rollers (161) (162) (163) and (164) for each color, and inking rollers (171) (172) (173) and (174) for each color.
When the carriage (148) is driven from the right side to the left, the blanket cylinder (150) is raised to separate from the surfaces of the tables or beds (141) - (144), and the damping rollers (161)-(164) and the inking rollers (171)-(174) make contact their corresponding printing plates so that the damping water and the color inks are supplied to the plates. Next, when the carriage (148) is driven from the left side to the right side, each of the color pattern inks formed on each of the plates is transferred to the blankets (151) (152) (153) and (154) loaded on the blanket cylinder (150). A paper to be printed is loaded on the outer circumference of the printing cylinder (158) and during one revolution of the blanket cylinder (150), the printing cylinder (158) which contacts with the blanket cylinder (150) revolves four times, and the color pattern inks on the blankets (151) (152) (153) and (154) are transferred in order to the paper so as to print a four color image thereon.
FIG. 13 shows a two color offset proofing press of a flat table type disclosed in British Patent Laid-Open Publication No. 2024105. In a frame (118) of this apparatus there are provided two plate tables (112) and (114) on which printing plates are to be loaded; a paper table (116) on which papers to be printed are loaded; a damping device (132) and two sets of inking arrangements (134) and (136). Further, in a carriage (120) which travels on the frame (118) there are provided two blanket cylinders (122) and (124) on each of the outer circumferences of which a respective blanket is loaded, a set of damping rollers (126) and two sets of inking rollers (128) and (130).
When the carriage (120) is driven from the right side to the left side, then in the same manner as for the apparatus shown in FIG. 12, damping water is supplied from the damping roller (126) to the plates loaded on the tables (112) and (114), and a color ink is supplied from the inking rollers (128) and (130) to the corresponding plate(s). On the other hand when the carriage (120) is driven inversely, that is, driven from the left side to the right side, color pattern inks on the plates are transferred to the corresponding blanket of the blanket cylinder (122) or (124), and then such inks are transferred onto a paper on the paper table (116) in order. Thus, a two color image is printed.
In those above-described, various types of multi-color offset proofing presses, blankets for each color are pressed, in order, onto a paper to transfer each color pattern ink to the paper, and a four- or two-color image is printed. In this case, because the first color ink printed on the paper from the blanket of the first color contacts the surfaces of the second blanket and those of the following ones, a phenomenon in which inks on the paper are transferred to the surfaces of the following blankets, i.e., so-called "back trapping", occurs; accordingly, ink quality on the paper is remarkably depreciated, so that excellent ink quality for obtaining a desired printing result cannot be achieved.
In each of the above-described apparatuses, immediately before starting a printing operation only one color ink corresponding to each of the blankets is to be transferred, and each of these blankets is contacted, in order, with the same paper. For example, considering the first color ink, after such first color ink has been transferred from the first color blanket to the paper, but while still wet, the second blanket contacts the paper, so that a part of the first coloring on the paper is transferred to the second color blanket. Thus, the so-called "back trapping" phenomenon occurs. In the case of the paper contacting the third and the fourth color blankets in the same manner as described above, the first color ink on the paper is also transferred to the following blankets, so that when the fourth color image is printed, the quantity of the first color on the paper decreases below that necessary for obtaining a desired printing effect.
When printing of four colors on the first paper is finished, on the second, the third and the fourth blankets there still remains some amount of the first color ink contrarily transferred thereto from the first paper; however, the quantity of such ink is small, and the further the printing order proceeds, the smaller the quantity of such ink on the blanket becomes. Accordingly, even in the printing operation to the second paper, as well as in the above case, the first color ink is transferred to the second blanket and those of the following ones, so that insufficiency of the first color also occurs in a printed image.
Regarding the second color ink or the third color ink, conditions are quite similar as those of the first color ink, that is, the second color ink is contrarily transferred to the third and the fourth blankets, and the third color ink is contrarily transferred to the fourth blanket, so that quantities of these color inks are insufficient in a finished printing. Such phenomena as has occurred in the above-described cases in which quantities of color inks become insufficient in an early print do not occur after a number of papers have been printed. This is because, if quantities of color inks of the preceding order are saturated on the following blankets, printing conditions become stabilized. However, a considerably large number of papers must be printed to reach the stabilized condition. Usually, the number of color printings required from an offset proofing press is relatively small. Therefore, in preliminary printing until the offset proofing press reaches a stabilized condition, an unduly large number of papers and a large quantity of ink are needed, and this problem is further compounded by the need for troublesome preliminary working of the press.